<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>蒙版</title>
</head>
<body>
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>CSS布局</title>
    <style type="text/css">
        *{
            margin: 0;
            padding:0;
        }
        img{
            width: 380px;
            height: 240px;
        }
        .abc{
            width: 380px;
            height: 240px;
            background: black;
            opacity: 0.5;
            position: absolute;
            top: 0px;
            left: 0px;
            z-index: 2;
        }
    </style>
</head>
<body>
<nav>
</nav>
<div>
    <img src="http://climg.mukewang.com/58f829ca00010be703840240.jpg">
</div>
<div class="abc"></div>
</body>
</html>
</body>
</html>